The invention relates to silicon monolithic PN junction isolated integrated circuit (IC) devices. Specifically the invention relates to the zener diodes fabricated into such IC structures. In order to avoid the unpredictable effects of the silicon surface the diode breakdown is confined to a subsurface region. This acts to reduce diode leakage and noise and increase stability. The advantages of subsurface diode operation is set forth in a publication by Robert C. Dobkin in the Sept. 16, 1976, issue of ELECTRONICS on pages 106-112. The diodes described were employed in the well-known LM199 series devices available from National Semiconductor Corporation and others.
U.S. Pat. No. 4,127,859 issued to Carl T. Nelson on Nov. 28, 1978, and is assigned to the assignee of the present invention. Here a subsurface diode is created between a P++ IC isolation diffusion and an N++ diffusion of the kind employed in the fabrication of NPN transistor emitters. An N+ buried layer must be located at the substrate to epitaxial layer interface to limit the penetration of the isolation diffusion into the silicon substrate. The zener breakdown is confined to the central portion of the underside of the N++ diffusion. This structure suffers from the disadvantage that the zener action occurs in a very heavily doped silicon region and the crystal structure in this region is not of high quality. Furthermore, the structure involves substantial surface area and therefore cannot be made very small.
U.S. Pat. No. 4,079,402 to James L. Dunkley and James E. Solomon, issued Mar. 14, 1978, and is also assigned to the assignee of the present invention. Here a thin ion implanted region produces a subsurface zener diode action. The process is employed in those IC devices that typically employ ion implanted JFET devices. These devices are employed in the LF155 series IC's available from National Semiconductor Corporation and others.
The teaching in the above-referenced publication and the two patents is incorporated herein by reference.